This invention relates to a production method and production apparatus of a toner for developing electrostatic images. To state it in more detail, this invention relates to a production method and a production apparatus of a toner for developing electrostatic images wherein toner particles are obtained through a process of practicing the removal of impurities attached to the surface of toner particles by the concentration of the slurry composed of particles produced at the time of toner particle production, and to a toner for developing electrostatic latent images obtained by said production method.
In recent years, it is disclosed in the publication of the unexamined patent application 2000-214629 that, as regards a toner obtained by a polymerization process based on a suspension polymerization method or an emulsion polymerization method, because the particle diameter and shape can be controlled during polymerization process in an aqueous medium, it can be obtained a toner composed of toner particles having a small size, a uniform size distribution, and a round shape with no protrusion on the particles; because of its reproducibility of fine lines and a high resolution, it is remarked as a toner capable of reproducing small dots for a digital image.
As described in the above, as regards a polymerization toner, toner particles are produced by a polymerization process in an aqueous medium, and from particles produced in a slurry state, toner particles dispersed in an aqueous medium are obtained. In order to extract the produced toner particles from the aqueous medium, slurry concentration for removing water from the aqueous medium in a slurry state containing particles is repeated. In the aqueous medium which is a dispersion medium of the slurry of dispersed particles, impurities such as free releasing agent particles and particles of their decomposition products leaving from the surface of the toner particles are contained. Therefore, because these impurities are adhering on the surface of the toner particles separated from the aqueous medium, it is necessary that the toner particles after separation are well washed to remove these impurities from the surface of the toner particles.
In the publication of the unexamined patent application 2000-292976, it is disclosed a technology relating to a toner production method wherein washing of toner particles is practiced with supply of water until the electrical conductivity of the filtered liquid becomes under a specified value while slurry concentration by centrifugal separation is being carried out. Further, in the publication of the unexamined patent application 2001-249490, it is disclosed that the following operation is repeated plural times: that is, slurry containing toner particles is stirred in a container equipped with a stirring plane and a filter material with a washing liquid added, and the slurry is made to pass the filter material under the application of a pressure to remove the washing liquid.
However, according to these technologies, in the slurry concentration process, only it is practiced that, with a filter cloth sheet or a filter material provided in a centrifugal separation apparatus and a stirring container at the time the slurry concentration is carried out, a filtration operation using a filter material is repeated.
Further, the inventors of the present invention newly found that a toner obtained through a process of slurry concentration disclosed in the publication of the unexamined patent application 2000-292976 had its charging ability brought into a varied state after it had been subjected to a continuously repeated copying process exceeding one hundred thousand times, and it was very difficult to provide a stable toner which is not subject to the change in the charging ability in a mass continuous copying over one hundred thousand sheets accordingly. Further, this tendency was more strengthened in the case where an image formation over a long period of time in a high-temperature high-humidity environment was carried out, and poor charging, toner fusing to component parts, and generation of contamination were remarkably presented. In the latest days, there are image forming apparatus which carry out supply of documents through outputting a mass of documents having a number of pages and binding them to have a shape of a booklet, and it is no more a rare thing in the uses of users to practice a continuous copying operation reaching hundreds of thousands of sheets or millions of sheets.
Further, it is inferred that the above-mentioned change in the charging ability generated in a mass continuous copying process exceeding one hundred thousand sheets is perhaps due to the removal of impurities being insufficiently made from the surface of toner particles for a toner having been subjected to slurry concentration using a filter material. That is, it is inferred that the impurities, which are released from toner particles at the time the concentrated slurry is again dispersed to wash the toner particles after the slurry concentration is carried out, are drifting like floating objects in the liquid, and therefore, when the second slurry concentration after washing is carried out, the impurities drifting in the liquid as floating objects are trapped in the toner layer formed on the filter material, and remain on the toner particle surface having been subjected to the final washing and drying.
Hence, as regards a toner obtained through a process of slurry concentration using a filter material, impurities are easily trapped again on the surface of toner particles; however, for a conventional centrifugal separation means, a filter material has been essential and indispensable in order to capture particles with certainty at the time of concentration.
Further, the average particle diameter of a polymerization toner is smaller than 10 μm, and in order to cope with the reproducibility of fine lines of a digital image forming apparatus in recent days, it is further advanced to make particle size smaller. However, it has been known that the concentration of slurry composed of small-sized particles having a particle diameter smaller than 10 μm using a conventional centrifugal means or a decantation means was very difficult for the reasons of the performance of the apparatus etc.
Further, as regards a conventional centrifugal separation apparatus, it is known that owing to the load by the centrifugal force at the time of slurry concentration, a degradation of toner property such that breakage of toner particles and their becoming fragile are brought about or the uniformity of the orientation of polar radials on the surface of toner particles is destroyed is brought about; therefore, in a conventional centrifugal separation apparatus, the operation is carried out with the centrifugal force at the time of slurry concentration suppressed as much as possible. However, because of the centrifugal force being made small, the lowering of productivity is inevitable, and on top of it, the degradation of the toner property cannot be avoided even though the centrifugal force is made small; thus, although the stabilization of the quality of a toner based on slurry concentration has been an important problem to be solved urgently, it has been unable at all to find the clue to the solution of it.